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Introduction to Networks
and the Internet
CMPE 150
Fall 2005
Lecture 3
CMPE 150- Introduction to Computer Networks
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Announcements
• Textbook should be available at bookstore by
the end of the week.
• Textbook will be on reserve at the S&E
Library.
• Homework 1 is up.
– Due on 10.10.
• Readings:
– Tanenbaum Chapter 1.
CMPE 150- Introduction to Computer Networks
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Labs
• Alternatives:
–
–
–
–
Mon 4-6pm.
Wed 4-6pm.
Tue 4-6pm.
Thu 4-6pm.
Preferred
• E-mail to boice@soe and todd@soe with
your name and preference.
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Last class…
• Data networks.
– Components.
– Communication model.
– Key tasks.
• Types of data networks.
– Coverage.
– Connection.
– Topology.
CMPE 150- Introduction to Computer Networks
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More Concepts…
 Network protocols.
• Layering.
• Network/protocol architecture.
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Layering
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Layering
• What is it?
• Building complex systems is hard!
– Approach: “Divide and conquer”.
– Split job into smaller jobs, or layers.
• Analogy to other fields.
– Building a house: digging, foundation, framing,
etc.
– Car assembly line…
• Basic idea: each step dependent on the
previous step but does not need to be aware
of how the previous step was done.
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Analogy: Air Travel
• The problem: air travel.
• Decomposed into series of steps:
Arrival at airport
Departure from airport
Check-in
Baggage claim
Boarding
Deplane
Takeoff
Landing
Traveling
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Arrival
Departure
Check-in
Baggage claim
Boarding
Deplane
Takeoff
Landing
intermediate air traffic sites
Airplane routing
Arriving airport
Departing airport
More on the air travel analogy…
Airplane routing
Traveling
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Protocol Architecture
• Task of communication broken up into modules
• For example file transfer could use three
modules
– File transfer application
– Communication service module
– Network access module
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Simplified File Transfer
Architecture
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A Three Layer Model
•
•
•
Application Layer
Transport Layer
Network Access Layer
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Network Access Layer
• Exchange of data between the computer and
the network
• Sending computer provides address of
destination
• May invoke levels of service
• Dependent on type of network used (LAN,
packet switched etc.)
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Transport Layer
• Reliable data exchange
• Independent of network being used
• Independent of application
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Application Layer
• Support for different user applications
• e.g. e-mail, file transfer
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Layered Protocol Design
• Layering model is a solution to the problem of
complexity in network protocols
• The model divides the network protocols into
layers, each of which solves part of the
network communication problem
– Each layer has its own protocol!
• Each layer implements a service to the layer
above
– Relying on services provided by the layers
below.
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Layers
• Layers are the different components that
need to be designed/implemented when
designing/implementing networks.
• Each layer responsible for a set of functions.
• Top layer relies on services provided by
bottom layer.
• Layer makes it service available to higher
layer through an interface.
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Layering: Logical Communication
E.g.: transport
• Take data from
application
• Add addressing,
information.
• Send result to peer.
• Analogy: sending a
letter.
data
application
transport
transport
network
link
physical
application
transport
network
link
physical
data
network
link
physical
application
transport
network
link
physical
CMPE 150- Introduction to Computer Networks
data
application
transport
transport
network
link
physical
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Layering: Physical Communication
data
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
CMPE 150- Introduction to Computer Networks
data
application
transport
network
link
physical
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Layers and Protocols
The relationship between a service and a protocol.
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Network/Protocol Architecture
• Set of layers, what their functions are, the
services each of them provide, and the
interfaces between them.
• A.k.a, protocol architecture or protocol stack.
• Examples:
– ISO-OSI 7 layer architecture.
– TCP-IP architecture (Internet).
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Protocol Data Units (PDU)
• At each layer, protocols are used to
communicate.
• At the source, control information is added to
user data at each layer, a.k.a., encapsulation.
• At the receiver, control information is stripped off
at each layer going up the stack, a.k.a.,
decapsulation.
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Operation of a Protocol
Architecture
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Example 1: ISO OSI
Architecture
• ISO: International Standards Organization
• OSI: Open Systems Interconnection.
Application
Presentation
Session
Transport
Network
Data link
Physical
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Layers of Interest in ISO Model
• Layer 7: Application
– Application-specific protocols (e.g. ftp, http, smtp)
• Layer 4: Transport
– Delivery of data between computers (end-to-end).
• Layer 3: Network
– Data routing across a network.
• Layer 2: Data Link
– Reliable transmission over physical medium.
• Layer 1: Physical
- Transmission of bits between two nodes.
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OSI Protocol Stack
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Example 2: TCP/IP Architecture
• Model employed by the Internet.
TCP/IP
Application
Application
Presentation
Transport
Session
Transport
Internet
Network
Access
Physical
CMPE 150- Introduction to Computer Networks
ISO OSI
Network
Data link
Physical
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TCP/IP Protocol Architecture
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Messages and Protocol Stack
• Example: Internet stack
M
Ht M
Hn Ht M
Hl Hn Ht M
source
destination
application
transport
network
link
physical
application
transport
CMPE 150- Introduction to Computer Networks
network
link
physical
M
message
Ht M
Hn Ht M
Hl Hn Ht M
segment
datagram
frame
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TCP/IP
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Encapsulation
Application data
TCP
header
IP
header
MAC
header
LLC
header
MAC
trailer
LLC PDU
TCP segment
IP datagram
MAC frame
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The ARPANET
The original ARPANET design.
IMP = Interface Message Processor (Honeywell DDP-316)
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The ARPANET Evolution
Growth of the ARPANET (a) December 1969.
(b) July 1970.(c) March 1971. (d) April 1972.
CMPE 150- Introduction to Computer Networks
(e) Sept. 1972.
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NSFNET
The NSFNET backbone in 1988.
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Internet Usage
• Traditional applications
(1970 – 1990)
• E-mail
• News
• Remote login
• File transfer
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Architecture of the Internet
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The Internet: Some Recent
History
• Between 1980 and 2000: the boom!
– Internet changed from small, experimental research
project into the world’s largest network.
– In 1981, 100 computers at research centers and
universities.
– 20 years later, 60M computers!
• Early 1990’s, the Web caused the Internet revolution:
the Internet’s killer app!
• Today:
– Almost 60 million hosts as of 01.99.
– Doubles every year.
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Types of Networks
• Circuit switching versus message switching.
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Circuit Switching
• Old telephone technology
• For each connection, physical switches are
set in the telephone network to create a
physical “circuit”
– That’s the job of the switching office
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Circuit Switching - Example
Physical copper
connection set up
when call is made
Switching offices
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Circuit Switching (cont’d)
• Switches are set up at the beginning of the
connection and maintained throughout the
connection
• Network resources reserved and dedicated
from sender to receiver
• Not a very efficient strategy
– A connection “holds” a physical line even
during “silence” periods (when there is nothing
to transmit)
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Message Switching
• No physical path established!
• Whenever sender has data to send, sends it.
• Data stored at first router then forwarded.
• Store-and-forward networks.
• Sharing by taking turns.
– Analogy: conveyor belt in a warehouse.
– Items are picked from the storage room and placed on the
conveyor belt every time a customer makes an order.
– Different customers may request a different number of
items.
– Different users’ items may be interspersed on the conveyor
belt (they are “multiplexed”).
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Packet Switching
• Upper bound on size of unit to be handled at
the network layer.
• Why?
– Fairness.
• What kind of implementation used by
Internet?
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Packet Switching Example
Header
Payload
A
C
C
D
C
D
B
CMPE 150- Introduction to Computer Networks
C
D
D
D
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Packet Switching
• Each packet is composed by the payload
(the data we want to transmit) and a header.
– The header contains information useful for
network layer functions.
– Contains:
•
•
•
•
•
Source (sender’s) address
Destination (recipient’s) address
Packet size
Sequence number
Error checking information
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Packet Switching (cont’d)
– The header introduces overhead, that is,
additional bits to be sent.
• Therefore, it is not wise to have packets that
are too small.
– What happens if the payload is just 1 bit?
– Addresses
• Each computer attached to a network is
assigned a unique number (called address).
• A packet contains the address of the sender
and the receiver.
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Packet Switching (cont’d)
• In general, packets need not be of the same size
– Maximum transmission unit (MTU)
– No minimum size
• But, header size is fixed (e.g., 20 bytes for TCP/IP).
• Original data chopped up into packets.
– The application (e.g., email) does not know that the
data to be transmitted is packetized.
– When packets are received, they are put together
before the application accesses the data
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Packet Switching (cont’d)
• What kind of delay should we expect?
– Time-division multiplexing: constant delay.
– Packet switching multiplexing: variable delay (it
depends on the traffic on the line).
• Conveyor belt example: if there are many
customers before you, you may have to wait more.
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Circuit Switching vs Packet Switching
Circuit switching
• Must set up a connection
(initial delay)
• Connection is reliable
• Resources are dedicated
– Therefore they are used
inefficiently!
CMPE 150- Introduction to Computer Networks
Packet switching
• Very small set-up delay
• Efficient shared use of
resources
• Possible congestion and
consequent packet
dropping
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Types of Network Services
• Connectionless versus connection-oriented.
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